1. Field of the Invention
The present invention relates to a substrate for an amorphous silicon semiconductor material and, more particularly, to a substrate for an amorphous silicon semiconductor material which is cheap and advantageously mass-produced and having improved short-circuit current and energy conversion efficiency. It is still more concerned with a substrate for a semiconductor solar cell.
2. Description of the Prior Art
Semiconductor devices which make use of an amorphous silicon (hereafter referred to as "a-Si"), such as a photosensor, a switch element and an image device etc. are well known.
Especially, the so-called a-Si solar cell which converts light to electrical energy by means of a photoelectric effect of a-Si is widely known to be used for consumer articles such as electric calculators and watches etc., or for industrial articles such as used for generation of electrical enery by means of solar light.
However, solar cells heretofore in use are high in cost and further cost reduction is required for large area system such as required for generation of electrical energy by means of solar light.
Until now, a-Si solar cells have been produced by, for example, decomposing monosilane (SiH.sub.4) by means of glow discharge to deposit a-Si on a substrate such as stainless steel etc. and then a transparent electroconductive layer such as SnO.sub.2 etc. or a Schottky barrier layer such as platinum etc. is provided thereupon, and further an electrode layer is provided thereupon.
Usually, the a-Si is used as a thin layer having thickness of around 1 .mu.m. Consequently, if there is a scratch or a projection on a surface of the substrate, it becomes a cause of pin-hole occurrence and the upper electrode and the substrate become short-circuited, thereby lowering the photoelectric conversion efficiency.
Accordingly, it is necessary for the surface of the substrate to be in a good state of smoothness without such a defect, that is, it must be subjected to a polishing treatment to acquire a mirror finish.
The polishing treatment, however, has technical difficulties in maintenance of parallelism of the substrate, removal of strains upon polishing, and selection of polishing materials and the like. Further, the yield is low. As a result, much time and high cost become necessary to obtain the desired mirror finish.
In order to improve such defects, it has been proposed to deposit SnO.sub.2, SnO.sub.2 and In.sub.2 O.sub.3, or the like on a stainless-steel substrate by means of vacuum evaporation, sputtering or chemical evaporation (Japanese Laid-Open Patent Application No. 69875/1981).
However, in the case of evaporation of a metal on a substrate, it is necessary to provide a very thick layer so as to form a flat surface. Further, the characteristics as a solar cell of the material thus obtained, for example, the short-circuit current value, are not good enough and improvement thereof is urgently required.